The invention relates to a single mode waveguide fiber having a refractive index profile, a core diameter, and a relative index which provide a high non-linearity coefficient. In particular, the high non-linearity coefficient is obtained together with pre-selected values for zero dispersion wavelength, cut off wavelength, and spectral attenuation in the 1550 nm operating window.
The xcex1-profile waveguide fiber core has been studied in considerable detail over the past several decades. The design of the first single mode waveguide to be manufactured included a step index profile in the central core region.
Core refractive index profile design has evolved as waveguide optical systems requirements have changed. The study of the core profile has been driven by the need for such waveguide features as:
positioning of cut off wavelength;
positioning of zero dispersion wavelength;
lower attenuation;
improved bend resistance; and,
lower total dispersion and dispersion slope.
More recently, the very high performance telecommunication systems, i.e., those which include multiplexing, high data rates, long regenerator spacing, soliton propagation, or optical amplifiers, have resulted in a broader study of core index profiles to include designs which have high effective area to minimize signal distortion and dispersion due to non-linear effects.
In certain devices, however, increase in non-linear index of refraction or decrease in effective area can improve performance. One notable case in which performance is enhanced by increased non-linearity is that of a wavelength conversion device based upon modulational instability.
What is required by devices which make use of highly non-linear waveguide fiber is that the non-linear waveguide retain such characteristics as those noted above. The difficulty of making non-linear waveguides is therefore compounded because increased non-linearity usually requires increased concentration of glass forming metal oxides, termed dopants, which alter the waveguide core refractive index. The increased dopant concentration results in higher attenuation and effects mode power distribution which in turn effects the waveguide properties required for efficient operation of a device using the non-linear waveguide. In particular, increased dopant concentration causes the zero dispersion wavelength to increase beyond the wavelength region useful for soliton propagation.
The effective area is
Aeff=2xcfx80 (SE2 r dr)2/(SE4 r dr), where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light. An effective diameter, Deff, may be defined as,
Aeff=xcfx80(Deff/2)2.
The relative index, xcex94, is defined by the equation,
xcex94=(n12xe2x88x92n22)/2n12, where n1 is the maximum refractive index of the index profile segment 1, and n2 is the refractive index in the reference region which is usually taken to be the minimum index of the clad layer.
For the particular profile described in this application, the core region has one segment. The notation xcex94o is used to describe the relative index of this single segment. The notation xcex94c is used to describe the relative index of the clad region.
The term refractive index profile or simply index profile is the relation between xcex94 % or refractive index and radius over a selected portion of the core. The term alpha profile refers to a refractive index profile which follows the equation, n(r)=n0 (1xe2x88x92xcex94[r/a]xcex1) where r is core radius, xcex94 is defined above, a is the last point in the profile, r is chosen to be zero at the first point of the profile, and xcex1 is an exponent which defines the profile shape. Other index profile shapes include a step index, a trapezoidal index and a rounded step index, in which the rounding is due to dopant diffusion in regions of rapid refractive index change.
The novel single mode waveguide of this application meets the need for a waveguide which is highly non-linear, but retains the required characteristics in terms of low attenuation in the 1550 nm window, properly positioned zero dispersion and cut off wavelengths, low, positive total dispersion, and low dispersion slope.
A first aspect of the invention is a single mode optical waveguide having a core region surrounded by a clad layer. The core region has a surface, a diameter, a relative refractive index, xcex94o, and a refractive index profile. The diameter of the core is measured from the central long axis of the single mode fiber to the core surface. The relative index of the core, xcex94o, is greater than xcex94c, the relative index of the clad. Both relative indexes are referenced to nc, the minimum refractive index of the clad layer.
The core region profile is an x-profile, for which the relative index is in the range 0.016 to 0.040, and the diameter is in the range 3 xcexcm to 8 xcexcm. Given this basic structure, two key waveguide fiber parameters, which serve to define the waveguide, are, a zero dispersion wavelength, xcexo, in the range of about 1500 nm to 1570 nm, and a non-linearity constant in the range of about       3    ⁢          xe2x80x83        xc3x97          xe2x80x83        ⁢          10              -        16              ⁢          cm2      W        ⁢          xe2x80x83        ⁢    to    ⁢          xe2x80x83        ⁢    13    ⁢          xe2x80x83        xc3x97          xe2x80x83        ⁢          10              -        16              ⁢                  cm2        W            .        ⁢      xe2x80x83  
The non-linearity constant is n2 in the equation for refractive index, n=no+n2 P/Aeff, in which P is transmitted power, no is the linear refractive index, and Aeff is the effective defined above.
This novel waveguide, when incorporated in a wavelength conversion device, improves the performance thereof. In an embodiment of this aspect, the waveguide core has xcex1 in the range of about 1.8 to 2.4, core diameter in the range 5 xcexcm to 6 xcexcm, and core relative index in the range of about 0.019 to 0.030. In this embodiment and in the first aspect of the invention, cut off wavelength is in the range of about 1400 nm to 1500 nm, attenuation at 1550 nm is not greater than 1 dB, the dispersion slope is in the range of 0.03 ps/nm2-km to 0.10 ps/nm2-km, and total dispersion over the wavelength range 1520 nm to 1600 nm is positive but not greater than about 2 ps/nm-km.
Calculations have shown that the required waveguide properties together with the required waveguide non-linearity can be realized when the xcex1 is very large, which means the xcex1-profile is essentially a step index profile. Thus, xcex1 values in the range of 1.8 to infinity are contemplated by the inventors. Thus an embodiment of the invention is a step profile or a rounded step profile. The xcex94 value and core diameter do not change in this embodiment of the profile shape.
In another embodiment of this first aspect the clad layer refractive index profile is flat and has a constant refractive index equal to the minimum index, nc.
The invention may also be described solely in terms of a particular geometry associated with a particular a of the waveguide xcex1-profile. Thus, the invention is a single mode optical waveguide fiber having a core and a clad as described in the first aspect of the invention above. The xcex1 of the core region profile is in the range of about 1.8 to 2.4, the relative index of the core is in the range 0.019 to 0.030, and the core diameter is in the range 5 xcexcm to 6 xcexcm. In an embodiment preferred because of simplicity of manufacture, the clad layer refractive index profile is flat and has a refractive index equal to the minimum clad index, nc. Here again, higher values of xcex1 are calculated to be effective in producing a waveguide having all the required properties. Thus, step index and rounded step index are also proper descriptors of the profile shape of the novel waveguide.